1. Field of the Invention
The present invention relates to a method for fabricating a color filter substrate for use in a color liquid crystal display (LCD) and other display devices.
2. Description of the Related Art
LCDs are relatively small, thin and lightweight display devices with comparatively low power dissipation. By taking advantage of these features, LCDs are currently used extensively in a broad variety of electronic appliances. Among other things, active-matrix-addressed LCDs with switching elements are used particularly widely in office automation (OA) appliances such as personal computers, audiovisual (AV) appliances such as TV sets, and cell phones. Meanwhile, the size, definition, effective pixel area ratio (i.e., aperture ratio), color purity and other quality parameters of LCDs have recently been increased or improved significantly.
The structure of a normal active-matrix-addressed LCD will be described with reference to FIG. 15, which is a cross-sectional view thereof.
As shown in FIG. 15, the LCD 350 includes an active-matrix substrate 10 and a color filter substrate 300, which are arranged so as to face each other, and a liquid crystal layer 20 provided between these two substrates 10 and 300. Also, as viewed perpendicularly to the principal surface of any of these substrates, the LCD 350 has an (effective) display area and a non-effective-display area (i.e., picture frame area) that surrounds the display area.
The active-matrix substrate 10 includes a transparent insulating substrate 12 made of glass, for example, gate bus lines (not shown) to supply gate signals therethrough, source bus lines 14 to supply data signals therethrough, active components (not shown) such as thin-film transistors (TFTs) and transparent pixel electrodes 16. The gate bus lines, source bus lines 14, active components and pixel electrodes 16 are all provided on the substrate 12. The transparent pixel electrodes 16 are arranged in a matrix on the display area.
The color filter substrate 300 includes a transparent insulating substrate 302 of glass, for example, a color filter layer 390 consisting of red color filters 340, green color filters 350 and blue color filters 360, a light shielding layer 330 including a plurality of light shielding portions 330A and 330B, and a counter electrode (not shown). The color filter layer 390, light shielding layer 330 and counter electrode are all provided on the substrate 302. The red, green and blue color filters 340, 350 and 360 are arranged so as to face their associated transparent pixel electrodes 16 on the active matrix substrate 10. The light shielding layer (i.e., black matrix) 330 is arranged such that the light shielding portions 330A and 330B are arranged in the gaps between the respective color filters and in the picture frame area.
In this manner, a color filter substrate normally includes the three types of color filters, namely, red, green and blue color filters. Accordingly, the white chromaticity, for example, is determined by the intensities of light rays that have been transmitted through these three types of color filters. For that reason, to adjust the white chromaticity arbitrarily without decreasing the color purities of the respective colors, a color filter substrate, in which the ratio of the areas of three color filters of each set is adjusted appropriately (i.e., the ratio of the areas of color filters in the three colors is not one to one to one), was developed (see Japanese Laid-Open Publications Nos. 3-198027, 7-159771 and 11-174430, for example).
However, in fabricating a liquid crystal display device by bonding a color filter substrate and an active-matrix substrate together, misalignment may happen. For example, if a color filter substrate, on which color filters are arranged in stripes, has failed to be aligned with an active-matrix substrate in the pixel row direction (i.e., perpendicularly to the pixel column direction), then source bus lines, extending along the pixel columns on the active-matrix substrate, will overlap with the respective color filters. As a result, some of the color filters cannot be used for display purposes anymore.
In a color filter substrate including color filters with an appropriately adjusted area ratio, the ratio of the areas of color filters in the three different colors is adjusted by changing the shorter side length (i.e., the width or the length as measured in the row direction) of striped color filters in one color from that of color filters in another color (see FIG. 1 of Japanese Laid-Open Publication No. 11-174430, for example). However, if the misalignment described above happened while such a color filter substrate and an active-matrix substrate are being bonded together, then the percentage of the shielded area of a color filter in one color, which has overlapped with the source bus line, to the overall area thereof would be different from that of the shielded area of a color filter in another color to the overall area thereof. The shielded area percentage should change because the color filters of the three types have mutually different areas. For example, the color filters having the smallest area of the three types should have the highest shielded area percentage.
Accordingly, even if the ratio of the areas of color filters in the three different colors has been appropriately adjusted so as to achieve desired white chromaticity, the misalignment would make the actual ratio of color filter areas contributing to a substantive display operation different from the desired one, thus increasing or decreasing the actual white chromaticity from the desired one.
To prevent the actual color filter area ratio from shifting from the desired one even in case of such misalignment, the light shielding portions may have an increased width according to a proposed method. However, such a method is not preferred because the effective pixel area ratio (i.e., the aperture ratio) would decrease in that case.
For the sake of simplicity, the problems of the prior art have been described as to a liquid crystal display device including striped color filters. Actually, though, those problems may arise not only in the display device described above but also in any other type of display device in which the misalignment of a color filter substrate from the other substrate affects the pixel aperture ratio.